Adequate internal sizing of alkaline papers is an important issue for most papermakers. Early development of cellulose reactive sizing agents resulted in poor control of sizing with excessive amounts of sizing agent used, resulting in increased wet-end deposits, press picking, and in coefficient of friction problems with the paper surface. Problems still occur, mainly due to the overuse of sizing materials. The problems are caused by high surface area materials (e.g., filler and fines) found in the wet-end, which adsorb the size and render it ineffective.
The purpose of internal sizing is to impart resistance to liquid penetration to the sheet. Internal sizing, along with sheet porosity (which is controlled at the size press), controls ink penetration in printing and writing papers, along with binder migration in coating basestock. The sizing of alkaline papers with cellulose reactive sizing agents or "synthetic sizes" has been established for more than 30 years. Two synthetic sizes presently in commercial use, alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA), impart sizing to the sheet by means of a chemical reaction (covalent bonding) with the hydroxyl groups of cellulose fiber.
All commonly used untreated fillers (e.g., clay, titanium dioxide, calcium carbonate) are known to have a detrimental effect on sizing. Studies of alkaline papers filled with various types of calcium carbonate have revealed strong inverse correlations between filler specific surface area and internal sizing values in the sheet measured by the Hercules size test (HST). In circumstances where increasing the filler content would be advantageous, associated sizing problems have occurred affecting sheet quality, machine performance, and runnability.